The field of the disclosure relates generally to optical communication systems and networks, and more particularly, to data compression, coding, and quantizers for analog and digital optical systems and networks.
Conventional hybrid fiber-coaxial (HFC) architectures deploy long fiber strands from an optical hub to a fiber node, and typically many short fiber strands to cover the shorter distances from the HFC nodes to a plurality of end users. Conventional Multiple Service Operators (MSOs) offer a variety of services, including analog/digital TV, video on demand (VoD), telephony, and high speed data internet, over these HFC networks, which utilize both optical fibers and coaxial cables, and which provide video, voice, and data services to the end user subscribers. The HFC network typically includes a master headend, and the long optical fiber carries the optical signals and connects the link between the headend, the hub, and the fiber node. Conventional HFC networks also typically include a plurality of coaxial cables to connect the fiber nodes to the respective end users, and which also carry radio frequency (RF) modulated analog electrical signals.
The HFC fiber node converts optical analog signals from the optical fiber into the RF modulated electrical signals that are transported by the coaxial cables to the end users/subscribers. Some HFC networks implement a fiber deep architecture, and may further utilize electrical amplifiers disposed along the coaxial cables to amplify the RF analog signals. In the conventional HFC network, both the optical and electrical signals are in the analog form, from the hub all the way to the end user subscriber's home. Typically, a modem termination system (MTS) is located at either the headend or the hub, and provides complementary functionality to a modem of the respective end user.
The continuous growth of optical intra/inter-data-center link, 5G mobile fronthaul (MFH) and backhaul (MBH), next-generation distributed HFC architectures and access networks, passive optical networks (PONs), and high-speed optical short-reach transmission systems with advanced multi-level modulation formats require an equivalent growth in the development of advanced digital multi-level modulation formats to process the vastly increased amount of data transmitted over the various networks. Presently, conventional deployments of 1G/10G PON systems using nonreturn to zero (NRZ) modulation are unable to meet the growing capacity demand to deliver future high-speed data and video services.
The growth of the optical link and network architectures has been matched, and in many ways outpaced, by a continuously-growing demand on high-speed Internet, high-definition TV, and real-time entertainment services, which has created an additional challenge for future broadband access networks. These emerging new services, which include virtual reality and 5G, are rapidly depleting the bandwidth resources of existing PONs, MFH, and HFC networks, where upgrades to system capacity and spectral efficiency is urgently needed.
Some conventional communication networks operate according to DOCSIS 3.1 specifications, which are standardized, and feature orthogonal frequency-division multiplexing (OFDM) and higher order of modulations (>4096 QAM). However, although the DOCSIS specification provides higher flexibility and spectral efficiency, it also presents new technical issues and challenges. For example, the demanding carrier-to-noise ratio (CNR) specified by DOCSIS 3.1 specifications for high order modulations cannot not be supported by legacy digital-to-analog (D/A) or analog-to-digital (A/D) converters having resolutions and the range of 8-10 digits. Replacement of every legacy D/A or A/D converter at the customer premises of a user would incur a substantially high cost, and therefore there is a need in the field new algorithms that are capable of suppressing quantization noise without the McKinley impairing D/A and A/D performances. Additionally, the continuous envelope and high peak-to-average power ratio (PAPR) of OFDM signals render the signals vulnerable to nonlinear distortions in analog HFC networks.
Recent progress in advanced A/D and D/A quantizers and data compression techniques for transmitting OFDM signals is encouraging improvement to the transmission techniques for digital RFoG (D-RFoG) systems. Some quantizer or compression techniques utilize partial bit sampling (PBS), but results in rapid increases to the quantization noise when reducing the number of digits. Fitting based nonlinear quantization (FBNQ) operations are recommended by the standard of Open Radio Equipment Interface (ORI). However, the FBNQ algorithm is complex and time consuming because it needs to estimate the statistical characteristics from a large number of samples, which increases the system delay. Moreover, the accuracy of FBNQ seriously degrades when quantizing the amplitudes distributed outside the interquartile range (IQR). Some matured companding methods, including μ-law and A-law, for encoding acoustic signals have been used in tuning the quantization levels. However, the logarithmic compression function of these methods is not optimal to suppress the quantization noise of Gaussian distributed OFDM signals. Accordingly, new algorithms are needed for optimizing the non-uniformly distributed quantization levels.
The growing performance requirements of 5G new-radio (NR), high-speed Internet access, and high-resolution multi-media entertainment with virtual reality create even further challenges to future fiber-wireless integrated MFH. Recent 5G-NR specifications feature OFDM and higher order modulations (e.g., 256- and 1024-QAM). However, proposals to integrate these two technologies have resulted in new difficulties, such as high sensitivity to nonlinear distortions and increased requirements on high-resolution D/A converters, which limit the quality and transmission distance of analog radio-over-fiber (A-RoF) optical networks in MFH. However, digital RoF (D-RoF) networks have demonstrated greater compatibility with different formats, as well as greater suitability for the 5G-NR environment that includes a more diverse spectrum and services. D-RoF also utilizes high immunity to nonlinear distortions, and better capability for error-free transmissions through use of forward error coding/correction (FEC) techniques. Furthermore, by the additional use of data compression and advanced modulation formats, D-RoF is better able to mitigate bandwidth and efficiency, and better digitally transport high-quality wireless signals between the baseband unit pool (BBU-pool) and radio access units (RAU) with increased transmission distance and improved power budgets. Accordingly, the results would need to develop algorithms to improve the compression efficiency of D-RoF MFH for 5G-NR specifications.